Hemoglobin acts as a receptor for a variety of drugs and other compounds. Some of these have potential as antisickling agents or agents that promote delivery of oxygen to infarcted tissue or tumors, but so far their clinical application has foundered on their strong affinity for serum albumin. Their stereochemistry of binding to the recently solved structure of serum albumin will be explored with a view to lowering their affinity for it without lowering that for hemoglobin. Human globin can now be made in E.coli. This has opened the way to directed mutagenesis of hemoglobin, capable of exploring the evolution of its structure and of its specialized functions in different species, and of modifications designed to make it usable as a blood substitute. One possible strategy towards the prevention of sickling would be inhibition of the phosphoglyceromutase that converts 1,3- to 2,3- diphosphoglycerate. Horseradish peroxidase is widely used in diagnostic tests and could be adapted to more uses by directed mutagenesis, but its structure is unknown. Efforts are under way to determine the structure of both these proteins.